In 1991, a friction stir welding technique of joining metal materials such as aluminum alloys was established in the United Kingdom. This technique is for joining metal materials by pressing a cylindrical tool for friction stir welding having a small-diameter protrusion at a tip thereof against joint surfaces of the metal materials to be joined and rotating the tool for friction stir welding, generating frictional heat, and softening and plastically flowing the metal materials at a joint portion by the frictional heat (Japanese National Patent Publication No. 07-505090 (PTL 1)).
“Joint portion” herein refers to a joint interface portion where joining of metal materials by butting the metal materials or placing one metal material on top of the other metal material is desired. Near this joint interface, the metal materials are softened, plastic flow occurs, and the metal materials are stirred. As a result, the joint interface disappears and joining is performed. Furthermore, dynamic recrystallization occurs at the metal materials at the same time. Due to this dynamic recrystallization, the metal materials near the joint interface become fine particles and the metal materials can be joined with high strength.
When aluminum alloys are used as the above-mentioned metal materials, plastic flow occurs at a relatively low temperature of approximately 500° C. Therefore, even when the tool for friction stir welding made of inexpensive tool steel is used, little wear and tear occurs and frequent replacement of the tool for friction stir welding is unnecessary. Therefore, in the friction stir welding technique, the cost required to join the aluminum alloys is low. Thus, in place of a resistance welding method for melting and joining aluminum alloys, the friction stir welding technique has already been in practical use in various applications as a technique of joining components of a railroad vehicle, a vehicle or an aircraft.
At present, the friction stir welding technique is mainly applied to nonferrous metals such as an aluminum alloy or a magnesium alloy in which plastic flow occurs at a relatively low temperature. This friction stir welding technique is superior to the resistance welding method in terms of cost and time required for joining, strength of the joint portion, and the like. Therefore, there is a need for applying the friction stir welding technique to not only joining of the materials in which plastic flow occurs at a low temperature, but also joining of copper alloys or steel materials in which plastic flow occurs at a high temperature of 1000° C. or higher.
However, when the friction stir welding technique is applied to the steel materials, the tool for friction stir welding itself is exposed to a high temperature during joining. As a result, the tool for friction stir welding is plastically deformed, and a portion of the tool for friction stir welding that is in contact with the materials to be joined is easily oxidized and becomes worn, which leads to remarkably short tool life.
As an attempt to solve the above-mentioned problem, Japanese Patent Laying-Open No. 2003-326372 (PTL 2), for example, discloses a tool for friction stir welding, in which a portion of a surface of the tool for friction stir welding that is in contact with materials to be joined is coated with a diamond film, thereby increasing the surface hardness, suppressing welding, to the tool for friction stir welding, of low-melting-point light alloy component such as an Al alloy or a Mg alloy serving as the materials to be joined, and lengthening the tool life. According to the tool for friction stir welding as disclosed in PTL 2, the wear resistance of the surface thereof can be enhanced and the tool life can be lengthened.
However, the diamond film has a very high thermal conductivity. Therefore, a part of frictional heat generated due to rotation of the tool for friction stir welding escapes to the tool for friction stir welding side, and sufficient conduction of the frictional heat to the materials to be joined side becomes difficult. As a result, an enormous time is required from when a small-diameter protrusion of the tool for friction stir welding is pressed against the materials to be joined to when plastic flow occurs.
In the case of joining of the materials in which plastic flow occurs at a high temperature, the tool for friction stir welding must be rotated at a high speed in order to speed up temperature rise of the materials to be joined. Therefore, if it takes time before plastic flow of the materials to be joined occurs, it is impossible to enjoy such an advantage of the friction stir welding technique that cost and time required for joining can be reduced. As an attempt to solve the above-mentioned problem, Japanese National Patent Publication No. 2003-532542 (PTL 3) discloses a technique of providing a heat flow barrier such that heat is not conducted to a shaft portion of a tool for friction stir welding. Provision of the heat flow barrier as mentioned above allows frictional heat to concentrate on materials to be joined.
As another attempt to suppress deterioration of the surface of the tool for friction stir welding, Japanese Patent Laying-Open No. 2005-152909 (PTL 4) discloses a tool for friction stir welding, including an underlying layer provided on a base material, and an anti-adhesion coating made of TiN, TiAlN or the like and provided on the underlying layer. According to this tool for friction stir welding, adhesion of a metal component (aluminum) in materials to be joined can be prevented in spite of long-term use, and thus, stable processing can be continued.